This article, and several others (including the external Wikipedia entry, which is how I originally arrived at this article) link to an article about AppleDouble (http://support.apple.com/kb/TA20578) which no longer exists. It's pretty bad when Apple's own internal knowledge base is referencing dead links. Where has this article gone?
Apple clearly makes an effort to maintain and clean up and update the articles they have posted, as Apple views as appropriate. Based on comments from Apple folks over the years, existing support articles can drop offline for any of various reasons, ranging from retirement to errors to, well, whatever.
As for your question, the Apple Double article references OS X 10.0 and 10.1 and a dot-underscore ._ prefix that can sometimes arise, and the text of one version of the article can be read here: